1. Field of the Invention
The present invention relates to downhole tools used in oil and gas drilling operations, and more particularly, to means for generating electrical power for measurement-while-drilling (MWD) tools and processes used in such operations.
2. Background of the Related Art
The drilling of oil and gas wells typically involves the use of several different measurement and telemetry systems to provide data regarding the subsurface formation penetrated by a borehole, and data regarding the state of various drilling mechanics during the drilling process. In MWD tools, data is acquired by sensors located in the drill string near the bit. This data is either stored in downhole memory or transmitted to the surface using a telemetry means, such as mud flow telemetry devices.
Both the downhole sensors and the telemetry means of the MWD tool require electrical power. Since it is not feasible to run an electric power supply cable from the surface through the drill string to the sensors or the telemetry means, electrical power must be obtained downhole. The state of the art MWD devices obtain such power downhole either from a battery pack or a turbine-based alternator. Examples of alternators used in downhole tools are shown in U.S. Pat. No. 5,517,464, assigned to the assignee of the present invention, and U.S. Pat. No. 5,793,625 assigned to Baker Hughes. An example of an alternator-like, electrical torque-generator (for maintaining the angular orientation of a tubular member within a rotary steerable system) is disclosed in U.S. Pat. No. 5,265,682, also assigned to the assignee of the present invention.
Turbine-based alternators employ rotors having impellers that are placed in the high-pressure flow of drilling fluid (“mudflow”) inside the drill string so that the impeller blades convert the hydraulic energy of the drilling fluid into rotation of the rotor. The rotors rotate at an angular velocity (speed) that provides enough energy to the MWD tools to power the telemetry means and sensors, and in some cases other tools in the drill string bottom-hole assembly (BHA).
In most conventional designs, the shaft of the turbine (i.e., the rotor) is coupled to an alternator either directly or via a gear train, which may adapt the rotor's rotational speed for optimum operation of the turbine and alternator. The turbine shaft is supported by bearings. Typically, the shaft, bearings, gear train, and alternator are all housed in a pressurized oil chamber in order to function in clean and well-lubricated conditions. Since the turbine shaft is rotating in drilling fluid, a rotary seal is required to isolate the drilling fluid from the oil in the pressurized chamber. As the rotating velocity is high, the face of a typical rotary seal has to be lubricated by something other than the drilling fluid, since the drilling fluid contains erosive particles that will quickly ruin the rotary seal. This lubrication is achieved by ensuring a constant, low-volume oil leak from the chamber towards the rotary seal. This leak also prevents the flowing drilling fluid from invading the oil chamber, which is desirable since the cleanliness of the oil promotes a long operating life for the gears, bearing, and electrical components inside the oil (i.e., drilling fluid particles would erode moving parts and damage the alternator components.)
To achieve this controlled leakage of oil across the rotary seal, the oil in the chamber is slightly pressurized compared the mud pressure by a compensating piston equipped with a spring within the pressurized oil chamber. This piston is required to move over a certain distance over time, which makes the chamber longer and bulkier than it might otherwise have to be. Accordingly, it is well known that the compensating piston and rotary seal tend to make the downhole provision of electrical power expensive.
Furthermore, experience in the art has proven that the majority of the failures and maintenance costs associated with downhole alternators are due to the rotary seals and the length of the oil reservoir pressure-compensation system. A need therefore exists for a downhole power generation system that operates without the need for such components.
A need further exists for a downhole power-generation system that is capable of generating high power in difficult operating conditions, such as the presence of erosive drilling fluid or mud.
Definitions
Certain terms are defined throughout this description as they are first used, while certain other terms used in this description are defined below: “Downstream” means the direction in which drilling fluid is pumped to flow through a drill string, e.g., in the direction of the gravity vector in a vertical well section.
“Particles” means relatively heavy solids, some of which are designed to plug small holes, that are mixed or suspended in a drilling fluid. Particles can exhibit a wide range of sizes, and can include cuttings resulting from the drilling process as well as additives that are used to control the hydrostatic conditions of the borehole.
“Upstream” means the direction opposite to downstream.